Condition Assessment Methods for Reinforced Concrete Structures

While concrete used in wastewater applications can be quite resilient, proper design and corrosion protection are necessary to ensure the structure will last through its intended service life. Engineers commonly refer to American Concrete Institute (ACI) 350 Code Requirements for Environmental Engineering Concrete Structures for design standards in wastewater applications. The code provides concrete durability requirements, including:

A minimum 4,000 psi compressive strength and a maximum water-to-cement ratio of 0.45 because, in general, lower water-to-cement ratios will result in higher concrete strength, less permeability, and less durability issues.
A minimum of 2 inches of concrete cover over embedded reinforcing steel for corrosion protection.
Reinforcing steel spacing should not exceed 12 inches in flexural members (to mitigate cracking).

In highly corrosive areas, concrete linings or coatings serve as a first line of defense against corrosion. Two typical linings in wastewater applications are PVC sheet linings and spray-applied epoxies or polyurethanes. Unfortunately, even with protective design considerations implemented, deterioration can also occur at alarming rates in the right conditions. Common types of concrete corrosion found at wastewater treatment plants are carbonation, biogenic corrosion (H₂S), chloride attack, and erosion. You can read more about the mechanisms of concrete structure corrosion in this article.

As wastewater treatment facilities age, it is critical that the concrete structures are assessed at regular intervals to identify and locate areas of degradation early, while a repair is still a cost-effective solution. To many in the wastewater industry, Condition Assessment is defined as the engineering analysis of structures and facilities to determine the remaining service life. Condition Assessment includes the use of appropriate and established tools and techniques to measure the current condition of a structure or facility against industry standards. For V&A’s engineers, Condition Assessment is performed by collecting quantitative data in the field to complement visual investigations.

Field-based Condition Assessment methods should provide qualitative and quantitative answers to questions such as:

What is the depth of degradation of the concrete?
How much concrete cover remains over the reinforcing steel in the structure?
What is the remaining wall thickness of the pipe?

So how do technical experts go about assessing reinforced concrete structures?

*Concrete structure condition assessment*

The first step is to look for visual defects, such as clues that indicate isolated or widespread issues. The visual clues include exposed aggregate, yellowish concrete, cracks, spalls, and evidence of reinforcing steel corrosion (rust discolorations on concrete).

If visual defects are present, supplementing visual information with quantifiable data, such as dimensions, surface pH, depth of degradation, and reinforcing steel depth measurements, is advantageous. And in some cases, where structural rehabilitation may be required or where detailed remaining useful life analysis is being performed, you may want to obtain concrete cores.

Collecting Quantifiable Data

Identifying Concrete Cracking

Engineers performing field-based condition assessments where concrete crack defects are observed should identify the type of crack present. Two types of concrete cracks may be present in structures:

Tensile cracks (most common) are hairline shrinkage cracks formed during the initial curing process and during temperature fluctuations. The ACI 350 code requirement limiting the spacing of rebar is designed to restrict tensile cracking. Typically, tensile cracks have widths less than 1 mm to 3 mm, and lengths can range from a few inches long to the entire width of the concrete member. In general, tensile cracks are relatively harmless but may have long-term effects on durability if occurring too frequently.
Stress-induced cracks are structural in nature and should be identified for further investigation. A typical structural crack is a shear crack, which occurs diagonally at a 45-degree angle. Horizontal wall cracks on rectangular concrete members may also be a sign of overstressing. Any crack width measured a ¼-inch or greater may indicate a severe problem and should be addressed immediately.

Reinforcing Steel Corrosion

*Marking for delamination survey limits*

In addition to measuring crack defects, engineers should look for evidence of corrosion on reinforcing steel, which most likely shows itself as corrosion staining on immersed concrete. If staining is present, it may indicate shallow cover over the rebar, and a delamination survey (or concrete sounding) should be performed. Hollow sounds during a delamination survey are usually a dead giveaway that the reinforcing steel beneath is corroded. In areas where corroded rebar is suspected, a half-cell potential test can be performed to produce a temperature map of corrosion activity occurring in the area.

Concrete Surface Testing

Corrosion surface testing is a multi-step assessment technique used during field-based structure evaluations. Concrete hardness and penetration are tested by removing loose, degraded material from the surface, then measuring the depth to sound concrete material. In areas where H₂S is not prevalent, the depth to sound material is typically less than 1/4-inch and within the “no scaling” to “medium scaling” range. Where H₂S is present, up to 2 to 3 inches of soft material may be removed before reaching sound concrete, which is classified as “severe” scaling per ACI 201R guidelines for performing concrete assessments.

Surface pH can indicate how corrosive the exposed environment is to the concrete. A pH range of 6-8 is considered normal surface pH for concrete in a submerged environment that is negligibly corrosive. When the pH becomes more acidic, it usually means that some extent of biogenic corrosion or chemical exposure has occurred. The lower the pH, the greater the intensity of biogenic corrosion, and concrete will usually turn to mush when the pH is in the 1-2 range.

Reinforcing Steel Scans

Surface penetrating radar (SPR) scans are used to measure the depth of rebar within the concrete. This measured value is important because it represents the amount of concrete cover that protects the reinforcing steel from corrosion. As mentioned, ACI recommends a minimum of 2 inches of coverage for water-retaining structures (with some exceptions). SPR scans are also used to measure the spacing of reinforcing steel which is compared to record drawings or ACI design requirements for flexural members for general compliance. Concrete member thickness can be estimated with SPR scans.

Concrete Coring

Concrete cores provide useful information on compressive strength and durability. Concrete durability is measured through various concrete, including pH at depth within the core, chloride concentrations taken at various depths, and through petrographic analysis. Data obtained from concrete core testing is used to estimate the remaining useful life of the concrete member or structure.

At V&A, we are passionate about condition assessment. Our field-based engineers assist water and wastewater clients across the country in evaluating treatment facility conditions and developing cost-effective solutions to mitigate damage caused by corrosion. We welcome your condition assessment questions in the comment section below.

Click here for more about our Condition Assessment Services.

Contact Us